Plant biomass is comprised of sugars and represents the greatest source of renewable hydrocarbon on earth. Unlike other renewable energy sources, biomass can be converted directly into liquid fuels. The two most common types of biofuels are ethanol (ethyl alcohol) and biodiesel. Ethanol is an alcohol, which can be produced by fermenting any biomass high in carbohydrates (starches, sugars, or celluloses) once fermentable sugars have been obtained from the biomass material. Sugars generated from degradation of plant biomass could provide plentiful, economically competitive feedstocks for fermentation to produce chemicals, plastics, and fuels or any other product of interest.
Fuel ethanol could be made from crops which contain starch such as feed grains, food grains, and tubers, such as potatoes and sweet potatoes. Crops containing sugar, such as sugar beets, sugarcane, and sweet sorghum also could be used for the production of ethanol. Sugar, in the form of raw or refined sugar, or as sugar in molasses requires no pre-hydrolysis (unlike corn starch) prior to fermentation. Consequently, the process of producing ethanol from sugar is simpler than converting corn starch into ethanol.
The yield and concentration of desired carbohydrates in plants are key determinants of the technical and economic feasibility of downstream industrial processes. However, the metabolic networks of plants for biosynthesis of sugars show substantial internal buffering and redundancy, with the consequence that alteration to a key gene in metabolism of a sugar commonly results in no useful change to the harvestable yield of the sugar (Moore, Australian Journal of Plant Physiology 22: 661-679 (1995); Nguyen-Quoc and Foyer, J of Experimental Botany 52: 881-889 (2001); Fernie et al., Trends in Plant Science 7: 35-41 (2002)).